Offshore wind turbines are being developed that instead of resting on fixed-bottom support structures have a floating support structure.
Several configurations have been proposed for the floating or buoyancy structures: many of these employ floater elements in the form of hollow floater tanks that in use are arranged substantially below the mean sea level and provide a buoyancy force to support the wind turbine. Ballast and/or mooring lines anchored to the seabed are provided for achieving stability.
In some of these floating wind turbines, the buoyancy structure is designed to provide an excess buoyancy force and is maintained floating under the mean sea level by taut mooring lines tensioned by the excess buoyancy force.
For example, concepts have been developed such as the “Taught Leg Buoy” (TLB) floating wind turbine, with a slender cylindrical buoy and at least two tensioned mooring lines, inclined relative to the seabed and connected to gravity anchors and to the buoy; or such as the “Tension Leg Platform” (TLP) floating wind turbine, in which the tensioned mooring lines are substantially vertical and are connected between gravity anchors on the seabed and arms or braces extending radially outwards with respect to the vertical axis of the wind turbine. The TLP arms may be part of the buoyancy structure, for example in the form of hollow spokes that extend radially outward from a hollow central hub, or may be arranged above the sea level, in which case the buoy may be a slender cylindrical tank like in the TLB concept.
The buoyancy structures of a floating offshore wind turbine (FOWT) is subject to several loads, such as for example the weight of the wind turbine itself, impacts, forces exerted by waves, currents and tides, and, depending on the configuration of the wind turbine, also aerodynamic forces associated with the wind, rotor rotation, etc. In the presence of such loads floating wind turbines may have a tendency to destabilize.
Furthermore, floating offshore wind turbines compared to their fixed substructure counterparts i.e. the monopile, jacket, tripod, or gravity based, may have a completely different dynamic response. Offshore wind turbines are highly dependent on the boundary conditions established by the water in which they reside and by the cables anchored to the seabed. On the one hand, these types of buoyancy structures are subject to relatively large displacements at the tower base (surge, sway, heave), which may affect the dynamic response of the nacelle-rotor-assembly due to elevated accelerations. On the other hand, they are subject to relatively large rotations (roll, pitch, yaw) at the tower base, which may compromise the structural integrity of both the buoyancy structure and the tower, with the danger of ultimate collapse.
The aforementioned loads may cause a FOWT to oscillate. In order to stabilize a FOWT with such a buoyancy structure, several solutions are possible. One example solution is to reinforce the buoyancy structure by adding ballast at the bottom of the floater tanks. Another example is to provide extra mooring lines which are put under tension by providing an excess buoyancy to the floater tanks. A combination of both solutions is also possible. However, these solutions increase material significantly. As the weight of the buoyancy structure goes up, so does the cost of manufacture and installation.
Alternative known strategies to reduce oscillations in a FOWT are based on the mitigation of loads by means of an optimized control of the wind turbine. In particular, it is known to pitch the wind turbine blades, so that certain loads (e.g. wind thrust acting on the rotor) are reduced. Nevertheless, these strategies have limited effect and can only mitigate oscillations resulting from some specific forces (not all).
It would be desirable to provide a floating offshore wind turbine in which the above mentioned drawbacks are at least partly solved.